Simple Webmachine Extension (3/4): ETags

This is the third post in a four-part series about extending a simple Webmachine resource. The first part discussed adding support for the HTTP method PUT, and the second part added basic authorization.

ETag

If I’m sharing management of a server with someone else, I want to be careful of overwriting that other person’s changes. For instance, I might only want to engage “DANGER_MODE” if I can be sure that “COAST=clear”.

I need to know that between my last GET and my next PUT, that the value of the “COAST” variable hasn’t changed. I can handle this simply by generating an ETag for the resource:

Now I can issue conditional requests. When I GET /_env the response will have an ETag header, which is reasonably guaranteed to change if the environment variables change. I can take that ETag and toss it back in an If-Match, and the request will only succeed if the environment variables are in the same state as I last saw them (because the ETag won’t match otherwise). That is: